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What is Dimmer Dot

Dimmer Dot

The "Dimmer Dot" is a Variable Transformer and provide continuously adjustable voltage A.C. supplies. In its "Auto Transformer" from (type-D, as manufactured by Green Dot Electric Limited), it is an efficient, reliable method of controlling A.C. voltage from zero to line voltage by amplitude modulation of the A.C. waveform. Since the Current, Power, Heat, Light and Motor speed are further functions of the voltage, the "Dimmer Dot" Variable Transformer finds its varied use. A few of the applications are

Voltage control in power supplies -as for Servo-type, Automatic Voltage Stabilizers, constant current supplies etc.

Voltage control for meter calibration, phase-angle control in the calibration of wattmeters and power-factor meters.

Motor speed control.

Control of electric heaters and ovens in laboratory, pilot plant, and production line.

Lighting control in theater, auditoriums, photographic studio and dark rooms.

The versatility of control are further high lighted in a separate publication "Tech Note" on "Dimmer Dot" Variable Transformers. The circuitories there in are only a few of the applications.

SALIENT FEATURES

The "Dimmer Dot" Variable Transformer, type-D consists of single layer winding on toroidal silicon-steel core. As the control knob is rotated, a carbon traverses the winding, tapping off a portion of the total voltage across the winding. The brush is in continuous contact with the winding, and the voltage between turns is always less than I volt, even in the largest model; in the smallest model it is only about 0.3 volt. The discrete voltage increments obtained as the knob is rotated are always less than the voltage between turns. Since the brush spans more than one turn, the change in voltage is practically continuous. The brush is so designed that excessive heating can not occur in the turns that it spans.

Dimmer Dot have many operating advantages over any other manually operated voltage control device for alternating-current use. Amongst the more important features are:

GOOD REGULATION - Output voltage is substantially independent of load.

SMOOTH CONTROL - The "Dimmer Dot" can be set very closely set to any output within its range.

HIGH EFFICIENCY - Very low losses under all load conditions.

LONG LIFE - With the exclusive "Dura Dot" contact surface, the life of the "Dimmer Dot" Variable Transformer is essentially the same as that of any standard fixedratio power trans.

LINEAR OUTPUT VOLTAGE - Output voltage is continuously adjustable from 0 to 112% of line by approximately 340 degrees and in certain models it is more in certain models. The dial reads directly in no load output voltage at rated line voltage or some dials are calibrated fo rpercentage of the full range control.

ADVANCED MECHANICAL DESIGN - Rugged construction - no delicate parts-two or more units can be ganged on same shaft-shaft is reversible for simple conversion from

CONSTRUCTIONAL FEATURES

Constructional features as different from other Variable Transformers lie in respect of INSULATION - that the insulation between windings and core is provided by mouldings so as to prevent any possibility of slackening of the turns during use and also to proved for a uniform hard bed under surface for the winding so as to make a strong base and under surface for long life of the track.

WINDINGS-are of high conductivity copper insulated with synthetic enamel of thepolyvinyl acetal-phenol formaldehyde resin type providing good space factor, extremely high abrasion resistance and having a good ability to resist the effects of prolonged heating. The windings are done by toroidal winding machines specially designed and developed by Green Dot during the past number of years to produce Constant winding tension and even pith. The winding are prevented from any possible movement during use by providing serrations on the winding rings and the further fixing the under pressure & use of high temperature bearing epoxies, through a special technique developed over years of experience.

BRUSH MECHANISM-"DIMMER DOT" Transformers are known for their trouble free brush gear. They have produced a self aligning brush mechanism which fulfils the following requirements.

- Minimum heating and wear

- Maximum contact area

- Constant contact resistance and losses

The temperature rise of brush mechanism and winding in any Variable Transformer, is dependent upon the contact pressure and the area of contact, which in turn decides the current in the short circuit, which in turn decides the current in the short circuited turn and brush face. "DIMMER DOT" brush mechanism have been developed to provide a constant brush contact area. The brush mounting holds the brush vertically in the direction of travel, so as to ensure that the same contact are is presented to the winding at all times with constant pressure. This enables the full contact are to be maintained, with resultant minimum contact temperature. They are provided with extra grip holder, greatly increasing their physical strength and offering great resistance to damage from shock and rough usage. The track cannot be shorted by metal brush holder fittings even in the worst conditions.

FRAMES - the wound core are held strongly and insulated from mounting fixture so as to proved minim high voltage standing of 2.5KV. The economised models are also available for OEM uses. The facilities can be provided for fitting stops and micro switches.

BEARINGS - the units are fitted with bearings, of bush type of nylon or glass filled nylon, for higher sizes, heat and light stabilized unaffected by atmospheric conditions, or sintered modified bronze or sintered bronze, self lubricating type as not to need lubrication through the life time of the product or the ball bearings as optional or the motorised unit. While ganging up the units of two or more numbers, the intermediate bearing guides are removed to provide rotary smooth operation. The Torque figures for various models can be had on request.

GENERAL - the "DIMMER DOTS”-are manufactured in certain fixed sizes determined by the voltage and current ratings. Type-D are manufactured in 0.7, 2, 4, 8, 15, 28, 40, 50, 60, 75, 100, 150 and 200Amps., and voltages of Input 270 Volts for 1Phase with taps at 0-30-240-270 voltages so that for 3 Phase operations, 3 single Phase unit can be connected in Star so as to provide for 470 Volts phase to phase connection facility.

Invariably all 3 Phase units are ganged assemblies of 3 single Phase units mounted on a common shaft. The facility exists for changing the shaft to make front shaft unit to back shaft unit suitable for back of panel mounting for 4, 8, 15 and 28 Amps units of single phase type. The 3 Phase units of back of panel and mounting have to be placed on supported structure at the back of the panel and the frond shaft can be brought through panel hole, obviously such arrangements can be for the air natural cooled unit. For the oil cooled units the containers have to be placed at the back of the panel and mounted or suitably held by keeping the unit with shaft in vertical position. 3 phase units are not available in 0.7 Amps. All the units are either fitted with screen printed insulating plates with schematic wiring diagram shown thereon or with a connection marking plate fixed there upon. The standard connection diagram and markings on the terminal plates are as follows

BACK OF PANEL - "DIMMER DOT" Units type D, of 4, 8, 15 & 28 Amps. ratings, single phase may be used either surface mounted or back of panel mounted. For ease of packing and transportation, at times, standard models are supplied for surface mounting, but the user can easily convert them to back of panel mounting, simply by removing the shaft locks or clamping rings by simply loosening he screws thereupon. The spindle or shaft is then adjusted after inserting from the other end, spacers/washers are suitably placed so that proper free movement of the moving arm without getting disturbed from the tail end and stopper is obtained with proper usable length of the Carbon from the Carbon Brush Holder available on track. Sufficient pressure from the spring holding the Carbon Brush must be ensured as available on the winding track.

METRIC CONVERSION - Small screws used in the models at present are B.A. sizes, 2, 4 & 6 B.A. having been used at small paces and conversions are being carried out to metric size. The changes are being carried out in a phase manner & is being intimated by sales department separately.

TRACK PREPARATION - "DURA DOT" track-" Green Dot " has developed over the years sophisticated track preparation method which provide fine tracks, designed to providing close tolerance surfaces. The optically ground tracks are plated with a hard silver alloy, ensuring constant resistance. After a quarter of million operating cycles, the Brush wear is only approximately 0.001 inch. This will allow for approximately 30 millions operating cycles before Brush wear demands a replacement Brush. "DURA DOT" contact surface is one of the most exclusive achievement made by "Dimmer Dot" for greater reliability, less maintenance and longer life of the "DIMMER DOT" variable Transformers. The contact surface eliminates all oxidation problems involved in the use of a carbon brush and carecopper contact and have the further following features to claim

LONGER LIFE - essentially the same as that of any fixed ratio transformers. Failures in any "DURA DOT" Dimmer are almost unknown. They are caused only by some factorswhich affect any standard fixed ratio transformer: such as due to failure of insulation because of prolonged over extended periods at currents in excess of rated. HIGH INITIAL SURGE CURRENTS- all Dimmer with "DURA DOT" contact surface will withstand initial surges ten times the rated current; they do not have to derated even for incandescent lamp loads (with may draw up to nine times rated current when first turned on)

OVER LOADS - the time current curve given in these pages is the safe allowable overload for "DURA DOT" Coated Dimmer-these overloads are considerably higher than the older types of Dimmers.

LESS MAINTENANCE - under normal conditions, maintenance of "DURA DOTS" is negligible-an occasional wiping of the contact surface with an alcohol-moistened rag being all that is required-the "DURA DOT" contact surface is subject to no deterioration when "DIMMER DOT" is operated with their rated loads.

DIMMER DOT TECHNICAL SPECIFICATIONS :

RATING

Ratings quoted are for use in ambient temperature from -20'C to +50'C. Current derating figures for temperatures above 50'C are as follows :

55'C

60'C

65'C

70;C

0.98

0.87

0.78

0.68

Generally, when the operation is required in ambient temperatures higher than 50'C output current must be reduced according to the curve shown below:

The current that may be drawn from a "Dimmer Dot" transformer through the brush is not constant as in fixed auto wound transformers, but varies according to brush position. At 50% and 110% of the line voltage, winding heat is the current limiting factor. Near zero and line voltage, winding heat is low and brush heat becomes the current limiting factor. Both factors operate at intermediate brush positions. This is common to all continuously variable "Dimmer Dot" Transformers. The "Dimmer Dot" ratings, in terms of "Rated Currents" "Maximum Currents" etc. should be fully understood for optimum results.

CURRENT RATINGS:

It is general practice to specify the current rating of Variable Transformer by two values, the rated current and the maximum current. Dimmer Dot have two current ratings because in any adjustable auto transformer, heating of the winding varies with the brush setting. To limit these useful devices to the current that can be drawn at any setting (RATED CURRENT) places an undue restriction on the capabilities. Most load (possible exceptions are some kind of motors and incandescent lamp loads) which draw MAXIMUM CURRENT at line voltage may be safely regulated from zero to line voltage without danger of overload. This MAXIMUM CURRENT is generally 20 to 30 percent above the RATED CURRENT and this concept may be safely used to full up the gap in the capacities of the RATED CURRENT of the various model available.

RATED CURRENT :

The rated current can be drawn at any brush setting. When the over-voltage connection is used, the load should not be drawn more than rated current.

MAXIMUMCURRENT:

May only be drawn near zero or near maximum output voltage when t h e Dimmer Dot transformer is line connected, i.e. when connected for an output voltage range of zero to line voltage. See Current Rating Curves.

The Maximum Current can only be drawn in line-voltage connection with the listed (or lower) input voltage across the whole "Dimmer Dot" winding (e.g. 240 volts on a 240/270 volt unit), and can only be drawn near zero or at or near full output voltage. A load having linear voltage/current load, line (constant impedance load drawing maximum current at the line voltage can be controlled from zero to line voltage. The knowledge of the maximum current and the rated current for the particular end application can be useful for filling in the capacity gap for the "Dimmer Dots" in the range and thus intermediate current requirements can at time be met from the particular rating valued "Dimmer Dots".

A "Dimmer Dot" transformer connected for over voltage provides an output of 12 % above the supply voltage on load. Most models have terminations for line voltage or over voltage connection, at the option of the user. Rated' current only may be drawn when over voltage connected.

OVERLOAD PROTECTION:

Although 'Dimmer Dot" are designed for arduous industrial use, damage will occur if they are subjected to short circuit or continuous severe overload.

It is therefore recommended that protective devices be fitted such as a circuit breaker in the input circuit & and H.R.C. fuse in the load circuit. It is normal practice to provide a protective device in the input circuit.

Improved core materials, permitting higher flux densities than was possible in the past, allow high inrush transient surges of up to 10 times the rated current to occur at switch on. This is not harmful except H.R.C. fuses or protective devices of instantaneous operation. It is therefore recommended that input protection "Dimmer Dot" is obtained by employing slow blow fuses or circuit breakers with slight operating delay. Input protective devices should be capable of handling 1000% overload for one cycle of the supply frequency.

It is not always appreciated that a variable transformer with an input voltage of 250 V and delivering say 10 amps to a load at 50 V (0.5 KVA), will draw a current of only 2 amps from the supply mains. It this instance, a 10 amp. fuse laced in the input circuit would not rupture until the load current reached 50 amps . It is obvious, therefore, that input protective device afford little protection to the transformer against overloads at low output voltage. The solution is, of course, to fuse the load circuit with an H.R.C. fuse link.

W A R R A N T Y

Green Dot
Electric Limited

Green Dot Electric Limited (the "Company"), New Delhi, warrants to the first end user purchaser (the "Purchaser",) of equipment manufactured by the Company that such equipment, if new unused and in original unopened cartons at the time of purchase will be free from defects in material and workmanship under normal use and service for a period of one year from date of delivery for the Company or for a period of one year form the date of delivery from the business establishment of an authorised dealer of the Company in the event that the equipment is purchased from an authorised dealer.

THE COMPANY'S OBLIGATION UNDER THIS WARRANTY SHALL BE STRICTLY AND EXCLUSIVELY LIMITED TO REPAIRING OF REPLACING AT THE FACTORY OR A SERVICE CENTRE OF THE COMPANY, ANY SUCH EQUIPMENT OR PARTS THEREOF WHICH AN AUTHORISED REPRESENTATIVE OF THE COMPANY FINDS TO THE DEFECTIVE IN MATERIAL OR WORKMANSHIP UNDER NORMAL USE AND SERVICE WITHIN SUCH PERIOD OF ONE YEAR. THE COMPANY RESERVES THE RIGHT TO SATISFY SUCH OBLIGATION IN PULL BY REFUNDING THE FULL PURCHASE OF ANY SUCH DEFECTIVE EQUIPMENT.

This warranty does not apply to any equipment which has been tampered with or altered in any way, which has been improperly installed or which has been subject to misuse, neglect or accident.

THE FOREGOING WARRANTY IS IN LIEU OF ANY OTHER WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATIONS, ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, and of any other obligations or liabilities on the part of the Company: and no person is authorised to assume for the company any other liability with respect to equipment manufactured by the Company. The Company shall have no liability with respect to equipment not of its manufacture. THE COMPANY SHALL HAVE NO LIABILITY WHATSOEVER IN ANY EVENT OF ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES, INCLUDING, WITHOUT LIMITATIONS, DAMAGES FOR INJURY TO ANY PERSON OR PROPERTY.

Written authorisation to return any equipment or part thereof must be obtained from the company. The Company shall not be responsible for any transportation charges.

IF FOR ANY REASON ANY OF THE FOREGOING PROVISIONS SHALL BE INEFFECTIVE, THE COMPANY'S LIABILITY IS BASED ON WARRANTY, CONTRACT, NEGLIGENCE, STRICT LIABILITY IN TORT OR OTHERWISE, SHALL BOT IN ANY EVENT EXCEED THE FULL PURCHASE PRICE OF SUCH EQUIPMENT.

Any action against the Company based upon any liability or obligation arising hereunder or under any law applicable to the sale of equipment, or the user thereof, must be commenced within one year after the cause of such action arises.